Name of this law ?

I saw a question on "Who Wants to Be a Millionaire" about a physics law where for example shaking a jar containing a big number of objects (like rocks, or chestnuts) will result in the bigger ones rising to the surface. I never got the chance to find the answer but does anyone know the name of this law so I can google it? or does anyone know how this works?

Unless the objects are very regular in shape, the large objects have larger gaps between them than the small objects, on average. The small objects can't make it between the gaps between small objects, but can make it between the gaps between large objects.